The major objective of this proposal is to test the hypothesis that thyroid hormones influence fetal lung growth and maturation, and pulmonary function in the normal and distressed neonate. The investigations are designed to test the following hypotheses: 1) That iodothyronines influence fetal lung growth, 2) that iodothyronines influence pulmonary surfactant maturation and synthesis, 3) that iodothyronines influence pulmonary function in distressed and normal pre-term and full-term neonates, and 4) that the methods used to determine metabolic clearance and secretion rates for thyroid hormones are valid. Studies in the surgically thyroidectomized ovine fetus have confirmed the lack of iodothyronines during the last trimester significantly delay lung maturation and surfactant synthesis. Infusions of thyroxine in both the athyrotic and euthyroid fetus can influence the timing of ovine fetal lung maturation.